fix: use a new managed channel builder when creating channels

[mutianf]

ChannelPoolDpImpl calls the channel supplier many times over a pool's lifetime — at warm-up, on load-driven resize, on capacity misses in newStream, and on channel recycles.

gRPC builders are mutating: intercept(...) appends to an internal list, and build() snapshots the current list into the new channel. So if anything installs an interceptor on the shared builder between builds, that interceptor accumulates. If the interceptor includes any headers, this causes all the headers to accumulate.

[gemini-code-assist]

Code Review

This pull request refactors SwitchingChannelPool to prevent interceptor accumulation by ensuring a fresh ManagedChannelBuilder is created for each channel supplier invocation, rather than reusing a single builder. It also adds a regression test to verify this behavior. The review feedback suggests improving the test teardown process by awaiting the termination of the server and executor to prevent potential thread leaks or port binding issues.